[MIPatternMatch]: Add ICstRegMatch

[Petar.Avramovic]

Summary

Matches G_CONSTANT and returns its def register.

[Petar.Avramovic]

Ping.

[foad]

Looks reasonable but should have at least one unit test.

llvm/include/llvm/CodeGen/GlobalISel/MIPatternMatch.h

93

Move all of this up to line 69 ish, just after m_ICst(int64_t &)?

[Petar.Avramovic]

Add unit test.

llvm/include/llvm/CodeGen/GlobalISel/MIPatternMatch.h

 ///{
 /// Convenience matchers for specific integer values.
 inline SpecificConstantMatch m_ZeroInt() { return SpecificConstantMatch(0); }
 inline SpecificConstantMatch m_AllOnesInt() {
   return SpecificConstantMatch(-1);
 }
 ///}
 
+struct ICstRegMatch {
+  Register &CR;
+  ICstRegMatch(Register &C) : CR(C) {}
+  bool match(const MachineRegisterInfo &MRI, Register Reg) {
+    if (auto MaybeCst = getConstantVRegValWithLookThrough(
+            Reg, MRI, /*LookThroughInstrs*/ true,
+            /*HandleFConstants*/ false)) {
+      CR = MaybeCst->VReg;
+      return true;
+    }
+    return false;
+  }
+};
+
+inline ICstRegMatch m_ICst(Register &Reg) { return ICstRegMatch(Reg); }
+
 // TODO: Rework this for different kinds of MachineOperand.
 // Currently assumes the Src for a match is a register.
 // We might want to support taking in some MachineOperands and call getReg on
 // that.
 
 struct operand_type_match {
   bool match(const MachineRegisterInfo &MRI, Register Reg) { return true; }
   bool match(const MachineRegisterInfo &MRI, MachineOperand *MO) {